The advent of digital recording techniques has enabled digital copies of digital content carried by recording media such as optical discs (for example DVDs) to be made with little or no loss of copy quality. This makes it easy for an unauthorised person to produce unauthorised copies of digital content-carrying recording media. Accordingly, copy-protection techniques have been developed. Most current copy-protection techniques exploit differences in the ways a legitimate player and an unauthorised copying apparatus or “ripper” would access a recording medium in order to defeat copying while preserving good playability. As an example, a legitimate DVD player follows a navigational path on the DVD that is defined by navigational commands and control data carried by the DVD whereas most unauthorised copying apparatus or rippers access the content of a DVD file-by-file or sector-by-sector. Accordingly, production of good quality or playable copies by a file-by-file or sector-by-sector unauthorised copying apparatus can be frustrated by adding subversive data (that is data that detrimentally affects at least one of the copying process, copy quality and playability of a copy) to a part of the digital data that does not form part of the navigational path of the DVD so that the subversive data is ignored by a legitimate player but detrimentally affects the production of an unauthorised copy. As another possibility, data may be altered to affect the digital sum value (DSV) of some of the digital data in a manner which does not affect a legitimate player but interferes with an unauthorised copying apparatus or “ripper”.
The above types of copy-protection may prevent digital copying of the content of an optical disc such as a DVD by a ripper that copies the data on a sector-by-sector or a file-by-file basis. However, other more sophisticated rippers may be able to parse navigation information contained on the optical disc so as to select the content data be copied. Thus, for example, so-called “IFO parsing rippers” are capable of parsing navigation information contained on a DVD and automatically selecting the “main title” or the “main program chain (PGC)” for ripping or allowing the user the possibility of selecting a particular title or a particular program chain from amongst those available.